1. Field of the Invention
The present invention relates to a toner for developing an electrostatic latent image. In addition, the present invention also relates to an image forming method, an image forming apparatus and a process cartridge, which form visual images using a toner.
2. Discussion of the Background
Recently, there are increasing needs for personal image forming apparatuses (such as copiers and laser printers) which are low-cost and small in size and which are environmentally friendly. In attempting to fulfill the needs, image forming apparatuses using a contact charging device which does not use a corona discharger have been investigated. Specifically, a charger (such as conductive rollers and brushes) is contacted with a surface of an image bearing member (such as photoreceptors) and applies a voltage thereto to charge the surface of the image bearing member so that the image bearing member has a predetermined potential. By using such a contact charger, a relatively simple and small-size charging unit, which can charge image bearing members by applying a relatively low voltage while producing a relatively small amount of ozone compared with cases where a charger using a corona discharger is used, can be used for the image forming apparatuses.
Contact chargers typically apply a DC voltage or a DC voltage overlapped with an AC voltage. In this case, residual toner particles which remain on the surface of the surface of an image bearing member and which have small particle size or light weight repeatedly perform abnormal charging and jumping movements in the vicinity of the contact point between a contact charger and the image bearing member. Therefore, problems such that the residual toner particles are electrostatically adhered to and embedded into the charger and image bearing member tend to be caused. Namely, contact chargers have a drawback in that the charging properties thereof easily deteriorate with time.
On the other hand, oil-less fixing devices have been typically used for color image forming apparatuses (such as color copiers and color laser printers). By using an oil-less fixing device, the fixing unit can be simplified and the running costs can be reduced because oil is not used. Toner for use in image forming apparatuses using an oil-less fixing unit has to have a good releasability and to produce color images with high glossiness. In general, it is impossible to impart a good combination of releasability and glossiness to toner, namely, the properties establish a trade-off relationship.
In attempting to solve the trade-off problem, various toners have been investigated. For example, in pulverization toners, new materials have been investigated for toner constitutional materials such as binder resins and waxes, and in addition wax dispersion methods for forming proper wax domains in toner particles have been also investigated. In contrast, toners prepared by a wet granulation method (such as polymerization methods and solution suspension methods) can include a relatively large amount of wax therein compared with pulverization toners. In addition, some of wet granulation methods can make it possible that a wax is included in a predetermined position of toner particles. Therefore, recently the manufacturing methods and the constitutional materials of polymerization toners and granulation toners have been actively investigated to develop toners suitable for image forming apparatuses having an oil-less fixing device.
In general, toners including a large amount of wax therein tend to cause a problem in that the wax exudes from the surface of toner particles, and the free wax released from the toner particles deteriorates the image qualities. Particularly, when the free wax is adhered to a developer bearing member or an image bearing member and forms a wax film thereon, various problems occur. Specifically, a background development problem in that the background area of images is soiled with toner particles is caused. In addition, when the wax film formed on an image bearing member is transferred to a charger, the charging ability of the charger deteriorates, and thereby a problem in that the image bearing member is defectively charged, resulting in deterioration of image qualities, is caused.
Further, since a granulation toner is manufactured (i.e., granulated) in an aqueous medium while using a surfactant, the resultant toner particles tend to have a relatively poor charge property compared with pulverization toners, and thereby problems such as the background development problem are easily caused.
In addition, when toner particles are prepared by a pulverization method, it is impossible to well disperse a relatively large amount of wax in the toner particles and therefore the particle diameter distribution of the wax particles (domains) in the toner particles is broad. Further, free wax particles are also included in the toner particles. Therefore, the toner has an undesired composition. As a result, the toner has a broad charge quantity distribution and thereby image quality problems due to defectively charged toner particles (such as the background development problem) are caused.
Further, when a wax is unevenly dispersed in toner particles, the toner particles tend to agglomerate. As a result, the toner has poor transferability, resulting in formation of hollow images (such as hollow character or line images).
In attempting to avoid such a filming problem, the following techniques have been investigated:    (1) suitable toner compositions are designed to prevent formation of free waxes and additives, which cause the filming problem;    (2) an abrasion agent which can remove a film is externally added to the surface of toner particles; and    (3) a lubricating material is externally added to toner particles so that a film forming material (such as waxes) is hardly adhered to image forming members such as chargers and photoreceptors.
With respect to the technique (2), techniques in that a particulate hard inorganic material having a particle diameter of hundreds of nanometers is added to toner particles as an abrasion agent have been proposed. For example, Japanese patent No. 2,656,230 (i.e., published unexamined Japanese patent application No. (hereinafter JP-A) 08-171230) discloses a technique of using cerium oxide as an abrasion agent. Japanese patent No. 3,407,545 (i.e., JP-A 10-10772) discloses a technique of using strontium titanate having an average particle diameter of from 200 to 800 nm as an abrasion agent. Although these abrasion agents can remove a film formed on a photoreceptor having a cleaning blade, a film formed on image forming members having no cleaning blade (such as contact charging rollers) is hardly removed and rather the abrasion agent is transported to the charging rollers, resulting in deterioration of the charging ability of the charging rollers. In other words, these abrasion agents contaminate charging members.
With respect to the technique (3), metal soaps are typically added as film preventing agents. Although metal soaps can prevent formation of a film on a photoreceptor, the charging properties of the toner deteriorate (for example, a charge-up phenomenon occurs in that the charge quantity of the toner excessively increases, resulting in increase of electrostatic adhesiveness of the toner to carrier particles, and thereby the image density is decreased).
JP-A 2002-31913 discloses a technique of using magnesium silicates (such as attapulgite, and sepiolite) as film preventing agents. These materials can prevent formation of a film on a photoreceptor in a cleaning process. However, since the materials have a high water content and thereby the toner is insufficiently charged, the background development problem, a toner leaking problem and a toner scattering problem, all of which are caused by the insufficiently charged toner, tend to occur.
JP-As 03-294864, 04-214568 and 05-165257 have disclosed techniques of using magnesium silicate treated with a silicone oil as film preventing agents. By using these materials, the fluidity of the resultant toners deteriorates, and in addition charge quantity thereof excessively increases. As a result, problems in that the toner is not well transported in a developing device and image density decreases occur.
JP-A 11-95480 discloses a toner using magnesium silicate as a film preventing agent, wherein the surface of the toner is covered with magnesium silicate at a covering rate of from 60 to 100%. When this toner is used as a negative toner, reversely charged toner particles are easily formed, resulting in occurrence of the background development problem. This is because magnesium silicate has a positive-charging property due to MgO (magnesia) as described in Journal of Japan Imaging Society Vol. 39, No. 3, p. 259.
JP-A 11-184239 discloses a toner including a titanate as a film preventing agent. Although titanate has good film preventing effect, charges of the toner easily leak because titanate has a low resistivity and thereby the background development problem, toner leaking problem and toner scattering problem tend to be caused. In addition, when titanate is transferred to a charging member, the charge imparting ability of the charging member deteriorates.
JP-A 2003-186240 discloses a toner including titania. Since titania has low resistivity and high dielectric constant, addition of a large amount of titania leak the charges of the toner, resulting in decrease of the charge quantity of the toner. In contrast, addition of a small amount of titania increases the charge quantity of the toner. In both cases, the background development problem, toner leaking problem and toner scattering problem are easily caused.
JP-A 2001-100453 discloses a toner including toner particles and an inorganic material which serves as an external additive and which includes at least an alkali metal salt of a fatty acid and a non-alkali metal salt of a fatty acid. However, this toner is insufficient with respect to the properties of background development and toner leakage.
Because of these reasons, a need exists for a color toner which has good charge properties without causing the filming problem and contaminating image forming members such as chargers.